The present invention relates to a refill lead writing instruments.
Writing instruments of the above mentioned general type are known in the art. In order to prevent breakage of a solid lead which is advanced from the writing pipe of a refill lead writing instrument or pencil, it is known to spring bias the clamping device. As a result, in the event of an excessively high writing force, the lead is moved back into the writing pipe and cannot be broken. For this purpose the spring force which acts on the clamping device is determined with respect to a continuous writing force and the strength of the lead. Because of this the applied writing force cannot exceed a predetermined value.
Refill lead instruments with spring biased clamping devices are known in many modifications. One modification in which the spring biasing of the clamping device is obtained by a pressure spring supported against the lead pipe is disclosed in the German document DD 43,171. A second pressure spring in this construction takes care of the closing of the clamping device which is composed of a conical clamping head arranged in a cylindrical clamping sleeve. A similar construction is disclosed in the German document DE-PS 3,910,546 which has a simple design and is easily mountable.
The German document DE-GM 8,903,990 discloses a writing instrument in which there is a single pressure spring which performs both the functions of the spring biasing of the lead against the writing pressure and the closing of the clamping chuck. All these writing instruments have the common feature that the lead projecting out of the writing pipe during exceeding of a predetermined abutment force is moved back to its position in the writing pipe until it is flush with its front end. In some cases it is however desired by the user to apply an especially high writing force to provide an especially intensive color by the lead. With a substantially vertical holding of the writing instrument this is also possible without breaking the lead. During working with the ruler, this is often desired and with a certain experience also lines with a high writing pressure can be applied.